Re: Seachem purigen
Hi fantails. Thank you for your post!
Purigen does not expire. Therefore, your Purigen from 2008 will be just as effective. You should only need 40mL of Purigen for your 40 gallon tank. Some hobbyists choose to use twice the recommended amount, which will allow for an extended period of time between regenerations.
Purigen could certainly help with diatom algae, since Purigen removes waste before it converts to nitrates. This will keep your nitrate level from increasing further, leaving less food for the algae to utilize to grow. You might also want to check phosphates, as diatom algae is usually an indicator that nitrate and phosphate are both elevated.
Let us know if we can help further!